The present invention relates to a prevulcanized tire tread strip for retreading and, more particularly, to the profile of the edges of the tread strip. The invention also relates to a tire obtained using this tread strip.
It is known that a tire consists of a flexible carcass which is roughly in the shape of a torus open on the axis side, and of a thicker tread strip which surrounds the periphery of the carcass and provides contact between the tire and the ground. The tread strip is subjected to more rapid wear than the carcass and reconstructing the tread strip when it is worn is less expensive than replacing the tire.
This reconstruction, known as retreading in the more general case in which it applies to the tread strip alone, calls upon several types of product and method; thus, the new tread strip may be raw or prevulcanized:
in the former case, the new tread strip, which is raw, and applied to the carcass beforehand, is vulcanized in a mold similar to the one used to manufacture the new tire; PA1 in the latter case, known as "cold" retreading, the prevulcanized tread strip is applied to the carcass, with the interposition of a layer of vulcanizable binding gum; the assembly is then usually surrounded, in whole or in part, by a leakproof membrane placed under pressure inside an autoclave to vulcanize the binding gum; the purpose of this membrane is to guarantee even application of a pressure of several bars, especially in the critical region of binding between the tread strip and the carcass throughout or during some of the vulcanizing operation. PA1 flat strips allow simple, reliable and effective retreading, but exhibit a number of drawbacks: reduction in the width of the tread strip by comparison with that of the new tire, mediocre appearance and the emergence of cracks during running; PA1 strips with "flaps" are provided along the sides with tapering extensions, which are also known as "wings", intended to envelop (in full or in part) the shoulders of the tire; in this way the final appearance of the retreaded tire is improved, but these strips display a serious drawback: during running, cracks emerge in the region of connection of the tread strip to the carcass, and these cracks spread dangerously, causing detachment of the tread strip or deep breaks in the carcass; PA1 the rings allow better application to the carcass and avoid the joining-together of the two ends of the tread strip, but they require more sophisticated means of fitting and do not prevent cracks from appearing in the connection region. PA1 "prevulcanized tread strip" is understood to be a tread strip which has already undergone a vulcanizing cycle, whether this cycle led to complete or to partial vulcanization; PA1 "carcass" is understood to be a tire without the tread strip, whether this tire is a new tire or a worn tire.
The present invention relates to this latter type of retreading.
The prevulcanized tread strip may be in one of a number of forms, for example, either very long and flat or preformed transversely curved strips, which are cut to the desired length at the time of retreading, or prepared rings of a given length:
Attempts at avoiding the appearance of cracks have been made by proposing the most tapered possible tread strip wings (EP 495 048, page 6, line 14) or by distributing the tensions in the region where the tread strip connects with the carcass, for example, by cutting circumferential grooves on the exterior sides of the tread strip (JP 6/191225). This last solution has reduced the flexural rigidity of the wings and thus has made them easier to apply to the shoulders of the carcasses of tires but has not lead to any significant reduction in cracking in the critical region of connection to the carcass.
Attempts have also been made to improve the quality of the bond between the tread strip and the carcass in this critical region after it was observed that the binding gum arranged between the carcass and the tread strip could run during vulcanizing until it covered part of the exterior surface of the wing of the tread strip, which part is not prepared for adhering securely to this binding gum. For example, U.S. Pat. No. 5,313,745 proposes to grind the two edges, interior and exterior, of the wings of the tread strips in order to improve their bonding to the binding gum. This solution substantially improved the resistance to cracking of the critical region, but this problem of cracks remains and hampers the development of the cold-retreading technique, even though this technique is very attractive in other respects.